Steel grit and method for manufacturing same



United States Patent STEEL GRIT AND METHOD FOR MANUFACTURING SAME Chalmer R. Cline, South Bend, Ind.

No Drawing. Application June 16, 1955,

Serial No. 515,969

7 Claims. e1. 51-409 This. invention relates to steel grit and to methods for use in the manufacture of same and it is an object of this invention to produce and to provide a method for producing steel grit for use in blast cleaningor blast processing of surfaces as by means of a centrifugal throwing wheel.

More specifically, it is an object of this invention to produce steel grit which is suitable for use in blast cleaning, peening and processing of surfaces; which has long life and wear in use for the preparation and processing of metal surfaces; which does not become embedded into the surfaces being processed; which does not contaminate the surfaces or break down into dust or the like substances, and which can be used to produce new and improved effects on the metal surfaces being processed.

It is a related object to provide a simple and efiicient process for the manufacture of steel grit to provide a process which makes use of materials heretofore considered scrap or waste in the manufacture of steel shot, a process which is capable of mass production techniques with relatively automatic equipment thereby to require a minimum amount of equipment and labor in the manufacture.

As used herein, the term steel grit is to be distinguished over shot formed of steel, iron or other metals in that shot generally exists in the form of pellets of rounded or curvilinear shape, whereas grit is formed of steel particles of small dimension having irregular shapes, and with edges that are relatively sharp by comparison with the generally rounded surfaces of shot.

Steel grit produced in accordance with the practice 0 this invention is believed to represent a new type of material for use in blasting of surfaces for cleaning and more particularly for surface treatment of metals to produce several different etches which resemble either one produced with conventional types of steel shot instead of grit, one obtained with conventional types of iron grit or one which is quite different than that obtained'with' conventional types of steel shot or iron grit depending upon the treatment given the steel grit prior to use. When used in surface treatment of metals, one variation or type of steel grit disclosed herein develops a matte type surface finish without leaving objectionable metal dust embedded in the surface of the work. Another variation or type of steel grit produced in accordance with the practice of this invention embodies the angularity characteristics of conventional iron grit, yet provides the type of finish available from steel shot, as distinguished from grit when used in blast cleaning. Still another variation or type of steel grit described herein produces a surface finish which resembles neither the typical shot type finish or the typical grit type finish but rather produces a finish somewhere between the two. Thus the product of this invention represents a new material combining the more desirable characteristics of steel shot with the more desirable characteristics of grit without bearing the majorundesirable characteristics of these types of materials such as heretofore have been used in blast cleaning and surface treatment of metals.

Steel grit manufactured in accordance with the practice of this invention makes use of steel shot as an intermediate and therefore provides an effective and economical means for utilizing the oversize material produced in the manufacture of steel shot, which oversize material has heretofore been rejected as waste material. Thus it is a further and important object of this invention, in the production of steel grit of the type described, to make use of scrap orwaste materials of little value and to convert these materials efliciently and inexpensively into a new and desirable product of high value.

In accordance with the preferred practice of this invention, shot is prepared of steel having a carbon content within the range of 1.10-1.40 percent by weight, a silicon content within the range of 0.40-0.90 percent by weight, and one or more hardening elements selected from the group consisting of manganese in amounts of 1.00-1.50 percent by weight, chromium in amounts of 0.50-5.0 percent by Weight, molybdenum in amounts ranging from 0.30-5.0 percent by weight, vanadium in amounts ranging from 0.05-0.40 percent by weight, nickel in amounts ranging from 0.50-2.5 percent by weight, and copper in amounts ranging from 0.20-1.00 percent by weight, the remainder being iron plus small amounts of permissible impurities.

By way of illustration, but not by way 'of limitation, the following is representative of a procedure for the manufacture of steel having the desired composition for use in the preparation of shot. Use may be made of an electric arc furnace in which the initial charge of scrap steel is reduced to a molten state after which ore (iron oxide) is added in amounts to obtain an oxidized state Within the furnace which is maintained throughout the heat. In practice, the iron oxide or ore is added before the bath reaches a temperature of approximately 2500-2600 F.

It is believed that oxygen present under the oxidizing conditions maintained within the furnace combines with silicon to form a silicon oxide which finds its way into the slag. This thereby tends to reduce the silicon content of the bath to about 0.10 percent or less. As 'will hereinafter be pointed out, it is desirable to maintain the silicon content at this low level during the heat because silicon tends to hold such gases as nitrogen and hydrogen which find their way into the shot or pellets formed as blow holes or hollows.

When the temperature of the bath reaches approximately 2700-2750" F., reaction begins to take place between carbon and iron oxide, first forming carbon monoxide and then carbon dioxide. The formation of carbon monoxide and carbon dioxide causes boiling to take place in the bath. During the boil, the entrapped gases and metal oxide inclusions are removed from the melt, producing a fluid metal having high surface tension.

After boiling to remove the entrapped or dissolved gases and the oxide inclusions, the bath is held at the elevated temperature for a refining period during which the composition of the slag is controlled by the addition of ferromanganese, mill scale, burned lime, fiuorspar and the like in the case of a basic furnace. or ferromanganese, mill scale, silica sand, lime and the like in the case of an acid furnace.

When the bath reaches pouring temperature at about 29003200 F., alloying ingredients are added such as by way of ferrosilicon and ferromanganese to adjust the bath to the desired composition. These materials function concurrently to de-oxidize the bath but it is desirable to make use of a strong de-oxidizing agent in addition to these alloying elements for securing the desired results.

For production of shot having rounded shapes and which are free of hollows formed by occluded gases, such as nitrogen and hydrogen, it is desirable, as previously pointed out, to maintain oxidizing conditions within the furnace until the addition of the alloying elements prior to pouring and for this purpose it is desirable also to reduce the silicon content in the bath to below 0.10 percent. The boiling period isintendedto remove the entrapped gases, the entrapped slag and oxide inclusions which may be formed by oxidation of such elements. as silicon, manganese, vanadium and the like in the melt;

p Unless removed by boiling, the entrapped. gases and the entrapped slag tend to remain in thebath and form impurities with corresponding development of weak spots in the shot that is formed.

The molten metal is preferably reduced to shot by the method described in the Pfaff- Patent No. 2,563,064. Briefly described, in the manufacture of shot, the molten metal is poured from the furnace through arefractory orifice of rectangular cross-sectionfrom which it falls vertically downwardly in a ribbon shaped stream.

The stream of molten metal is bombarded by blasts of water thrown substantially perpendicularly but somewhat downwardly across the path of the molten stream of metal from a centrifugal throwing wheel rotated at high velocity. Since the water is directionally discharged centrifugally from each blade of the rapidly rotating throwing .wheel, the molten metal stream is bombarded in rapid succession by layers of water to provide a pulsating action which operates to disintegrate the metal stream into relatively finely divided metal particles. While it is preferred to make use of a blasting wheel for bombarding the metal stream with increments of water, it willbe understood that disintegration of the molten stream of metal can also be accompanied by other methods such as by directing a solid stream of water into the falling stream of metal. The disintegrated particles of molten metal fall into a pool of Water for solidification. From the time that themetal stream is disintegrated until the metal particles are quenched in the pool of water, the high surface tension of the metal operates to round out the particles with the resuit that the product formed consists chiefly of rounded pellets of various dimensions.

\ The steel pellets that are formed can be used in accordance with the practice of this invention in the manufacture of steel grit. It is preferred, however, to separate the pellets according to size for the removal ofthose pellets of suitable dimension for use in the manufacture of steel shot. For blasting cleaning work, ten sizes in the range from approximately 0.007 inch to 0.111 inch in diameter are normally used. The oversize, which heretofore-has been considered a scrap material suitable only forremelt purposes in the manufacture of steel shot, can however be converted for beneficial use by manufacture of steel grit embodying the features of this invention. Thus, it is an important concept of this invention to provide a method for converting the oversize, heretofore considered a scrap material in the manufacture of steel shot, into a new, valuable and desirable product for use in the blast cleaning and treating of metal surfaces and it is a further concept of this invention to provide a method for efliciently and economically producing same.

Steel having a carbon content with the described range of 1.10-1.40 percent by weight is desirable when use is to be made of the formed shot of suitable dimension in the manufacture of steel shot. When the carbon content is reduced to an amount less than 1.10 percent by weight, greater proportions of martensite are formed during the initial disintegration and quench at the expense of included austenite. An increase in the amount of martensite has been found to be instrumental in the formation of an increased number of surface cracks that are formed in the pellets upon' quenching. Cracks in the surface introduce lines of weakness inthe pellets so that anexcessive number of cracks will cause disintegration of the shot upon impact, thereby materially to reduce the useful life of the pellets.

-Micro-examination of the shot has disclosed thatthe formation of martensite during the initial quenchappeared to be responsible for the formation of these surface cracks. Thus conditions efiective to minimize the formation of martensite and increase the amount of austenite formed upon quenching was found effective to minimize the formation of these undesirable surface cracks. By raising the amount of carbon, the ratio of retained austenite formed was also increased and shot substantially free of surface cracks was found to be available by the use of steel having a carbon content in excess of 1.50 percent. For example, shot formed of the steel having 0.77 percent carbon was found to contain 95 percent martensite and less than 5 percent retained austenite and a large number of surface cracks were developed on the surface upon quench. Shot formed of eutectoid steel having 0.85 percent carbon, containing about 95 percent martensite and less than 5 percent retained austenite, was also badly cracked. Steel formed with 1.50 percent carbon was found to contain about 10 percent martensite and about percent retained austenite. Shot produced with this steel was substantially free of cracks. Although the tendency to form cracks was increased as the carbon content was lowered, it was found that shot reasonably free of cracks could be made with steel having as low as 1.10 percent carbon. Because it was desirable, in the manufacture of steel shot, to produce pellets which contained a minimum number of surface cracks, it was necessary to limit the carbon content to a range in which the amount of martensite formed in the steel upon quenching is approximately 50 percent or less.

In the manufacture of steel shot, it is necessary to heat treat the pellets at an elevated temperature in the range of about 1800 F. for 1.5 carbon steel or higher and only about 1700 F. for steel having a carbon content of about 1.30 percent. When heated to 1800 F., the small particles of steel tend to agglomerate. Thus for use in the manufacture of steel shot, it has been found desirable to make use of a steel having a carbon content below 1.50 to avoid agglomeration and a carbon content above 1.10 to avoid the formation of cracks and in the preferred practice" the steel content is limited to within the range of 1.10-1.40 percent carbon, as previously pointed out.

When, however, use is made of the formed pellets solely for the purpose of preparing steel grit, the presence of cracks on the surfaces of the pellets is of lesser importance. In fact, surface cracks in limited amounts are desirable to the extent that less force is required to disintegrate or break down the pellets into the desired irregular particles of smaller dimension in the formation of steel grit. Thus the permissible carbon range in the steel employed for use in the manufacture of steel grit may be extended to below 1.10 percent down to about 1.00 percent but it is preferred to limit the carbon content in the steel to within the range of 1.l-l.4 percent. By way ofa preferred analysis, it is desirable to make use of a steel having the following composition:

Carbon 1.20:.10 Manganese 1.20:.10 Silicon 0.650.75 Sulphur Less than 0.05 Phosphorus Less than 0.05

Remainder iron, plus impurities.

The shot or pellets secured from the initial quench has been found to be too tough to be crushed or broken down into smaller particles of angular shape, referred to'herein as grit. The steel shot after initial quench contains a relatively high percentage of retained austenite and the shot hardness is in the range of 50 to 55 Rockwell C hardness. Rather than permit easy fragmentation, the shot tended to deform or flatten out under load or impact. When forces suflicient to crush the pellets are applied, the particles that are formed are characterized by having distorted shapes such as being in theform of flats or slivers as distinguished from the desired chunky and irregular grits.

In accordance with the practice of this invention, the oversize from the manufacture of steel shot or other steel shot manufactured for use in the preparation of grit is heat treated at a temperature above 1400 F. but below 1700 F. to increase the coarseness of the grain and to increase the hardness of the steel. When the shot following heat treatment is quenched in water, a micro-structure with excessive massive carbides is produced on the grain boundaries. This increases the hardness of the steel and it also produces a steel having much coarser grain. The net efiect is a steel which is capable of fragmentation under load to produce irregular, chunky, solid particles embodying the properties desired in steel grit. In practice, it is desirable to effect heat treatment under time and temperature conditions for raising the hardness of the steel pellets to above 58 Rockwell C and preferably to above 61 Rockwell C.

In the preferred practice, it is desirable to efiect heat treatment at a temperature within the lower range since a more friable and harder product is secured. The steel pellets are raised from approximately 53 Rockwell C up to approximately 63 or 64' Rockwell C hardness by a heat treating cycle of approximately'two and one-half hours which includes bringing the material up to 1450 F., giving it a soaking heat for about one hour at the same temperature and then quenching in water. A lesser increase in hardness is secured by heat treatment at higher temperatures. For example, the pellets are converted to a hardness of 58 to 61 Rockwell C by heat treatment at 1700 F. and to a hardness of about 61 to 63 Rockwell C by heat treatment at 1600 F.

Heat treatment of the shot is preferably carried out in a retort type of furnace heated to the desired elevated temperature and wherein the retort is mounted for rotational movement about its axis in a horizontal plane. When, as is preferred, the furnace is partially filled with the shot, the constant state of agitation to which the pellets are subjected upon rotation of the furnace causes the pellets uniformly to become heated up to the desired temperature and it prevents agglomeration of the pellets under the temperature conditions existing. During the reduced to the desired dimension. The oversize is re turned automatically to the feed end of the mill for additional crushing. The amount removed by the screens is made up by the addition of new shot to the feed so that the amount of pellets cycled through the mill will remain relatively constant.

For separation, use may be made of a bank of shaking screens having different screen sizes for separating the acceptable particles into lots, according to dimension.

cycle of heat treatment, an inert gas, natural gas or other carburizing gases may be introduced continuously into the retort to protect the shot from air or other oxidizing gases. This also helps to overcome the surface decarburization of the steel shot. Instead of making use of a rotatably mounted retort, other furnaces for heat treatment may be employed, such as a shelf type furnace having a reciprocating rake for agitation or use may be made of other conventional heat treating furnaces.

Heat treatment followed almost immediately by a water quench converts the steel from a predominantly retained austenite microstructure to a steel which is formed predominantly of martensite having a hardness and a coarseness of grain which enables fragmentation of the pellets as distinguished from distortion of the pellets under load.

Fragmentation of the heat treated pellets is preferably achieved by means of a ball mill. For this purpose, use is made of a ball mill of substantial dimension, such as a mill 5 feet in diameter packed with balls formed of forged steel and having a dimension of about 5 inches in diameter. Larger diameter mills and larger balls can be used where it is desirable to achieve more rapid breakdown of the shot by the greater impacts developed. Mills of smaller diameter with smaller balls may also be used but for the impacting forces necessary for fragmentation, it is desirable to favor the larger units. Instead of a ball mill, other crushers such as a hammermill, gyratory crusher or the like may be used for fragmentation.

In practice, the hardened steel shot is cycled continuously through the mill in a closed circuit with screens or other separating means for removing the particles The steel grits having a hardness of 60 64 Rockwell C can be used without further treatment in a number of applications where conventional iron grits are not suitable. Because of the hardness of the steel grits, it is difficult to blunt the edges on impact and therefore such hard steel grits are used primarily where it is desired to form a matte finish or an etched surface on the metal.

For example, use can be made of the hard steel grits in the blasting of steel surfaces which are to be glass enamelled because these hard steel grits produce a matte type finish to which the enamel is capable of developing a strong anchoring relation. When, instead, hard iron grits are employed for this same application, the hard iron grits become fractured upon impact during blasting and form into fragments having relatively sharp edges which become easily embedded into the surface. The iron grits also break down into powders and dusts which fin'd their way into the pores of the metal. Either of these conditions leads to poor enamelling. By way of comparison, the hard steel grits do not become embedded in the metal surface nor are the pores of the metal surface filled with a dust that is incompatible with the glass frit used in the enamelling operation. Instead of (fracturing into sharp slivers capable of becoming embedded in the surface, steel grit remains whole but the sharp edges become dulled or blunted sufiiciently to prevent their becoming embedded into the metal surface being treated.

It has been found that the life and the endurance of the steel grits can be greatly enhanced by heat treatment subsequent to fragmentation of the hard pellets to draw back the grits to a softer and more durable product. For a number of applications, it is desirable to draw back the steel to a hardness of about 43 to 47 Rockwell C and for this purpose the hard steel grits are heat treated at a temperature within the range of about 925-1000 F. This heat treatment takes the steel from a hard martensite to a tempered martensite with a concurrent reduotion in hardness from 60 to 64 Rockwell C to about 43 to 47 Rockwell ,C, depending upon the temperature and the time of heat treatment and the composition of the steel. The time of heat treatment to draw the steel back to a hardness of 43 to 47 Rockwell C depends greatly upon the temperature. For example, it will be sufficient to heat treat the grits for one hour at 1000 F. or for one and one-quarter hours at 925 F. Temperatures as low as 850 F. may be employed with correspondingly greater lengths of time for treatment but it is undesirable to make use of a temperature much in excess of 1000 F. because the time for heat treatment becomes so short as to interfere with the ability to develop a uniform hardness in the end product. The grits drawn back by heat treatment are cooled quickly, as by means of forced air draft in a rotary cooler.

For a number of applications, it is desirable to draw back the steel grits only partially from its 60 to 64 Rockwell C hardness to about 58 to 60 Rockwell C hardness. Drawing back the steel by this amount adds endurance life to the grits. In use, the edges become blunted but the grits retain their angular shape. Thus these as well as the harder grits are used when a matte type finish for etch is desired on the cleaned metal sur face. The softer grits for drawing back to a hardness of 43 to 47 Rockwell C round out more easily on impact with the result that the grits are quickly formed into rounded pellets in use and therefore can be employed in combination with shot or as or treatment of surfaces.

For certain' other applications where still longer 'endurance life of the grit is more important than fast clean-' ing ability, the grit can be drawn back to a still lower hardness in the 35 to 43 Rockwell C range. The same draw temperatures would be used as given above but the time of draw would be increased.

By heat treating the steel grits to an intermediate hardness between 47 and 58 Rockwell C, grits are obtained which will finish a surface somewhere between a shot finish and a matte or grit type finish.

It will be apparent from the foregoing description that I have provided :a new and improved means for producing steel grit capable of use in blast cleaning or treatment of surfaces to provide a new and different finish not available from steel shot or from iron grits. The product secured by the practice of this invention is capable of use to supplement the expensive steel shot used in the blast cleaning or finishing of metal surfaces thereby extending the overall yield of useful products from a given heat of steel and thereby lowering the cost of producing same.

It will be apparent also that the concepts described and claimed herein provide for the utilization of the over-size heretofore considered scrap in the manufacture of steel shot and the conversion of the oversize into a useful product capable of extending steel shot or capable of use independently of steel shot for the blast cleaning and finishing of surfaces.

It will be understood that changes may be made in the details of composition and in the details of procedure for manufacture of the steel grits as defined herein without departing from the spirit of the invention, especially as defined in the following claims.

I claim:

1. For use with centrifugal blasting wheels in the treatment of metal surfaces, crushed steel shot consisting of steel grit of small dimension and irregular shape having relatively abrupt edges and a hardness within the range of 35 to 64 Rockwell C in which the steel is a martensitic steel converted by heat treatment from an austenitic steel shot having from 1.00-1.50 percent by weight carbon, 0.40-0.90 percent by weight silicon, and a hardening element selected from the group consisting of 1.00-1.50 percent by weight manganese, 0.50- .0 percent by weight chromium, 0.305.0 percent by weight molybdenum, 0.05-0.40 percent by weight vanadium, 0.50-2.5 percent by weight nickel, 0.20-1.00 percent by weight copper and shot in the blast cleaning,

mixtures thereoflthe remainder being iron'plus small amounts of impurities. t

' 2. Steel grit as claimed in claim 1' in which the martensitic steel has a hardness within the range of to 64 Rockwell C.

3. Steel grit as claimed in claim 1 in which the steel is a tempered martensitic steel which has been drawn back by heat treatment to a hardness of 35 to 47 Rockwell C.

4. Steel grit as claimed in claim 1 in which the steel is a tempered martensitic steel which has been drawn back by heat treatment to a hardness of 47 to 60 Rockwell C. r

5. For use with centrifugal blasting wheels in the treatment of metal surfaces, crushed steel shot consisting of steel grit dimensioned to be less than 0.1 inch in the major direction and of irreguar shape having relatively abrupt edges and a hardness within the range of 35 to 64 Rockwell C in which the steel is a martensitic steel converted by heat treatment from an austenitic steel having the following composition-l.00-l.50 percent by weight carbon, 1.00-1.50 percent by weight manganese, 0.4-0.9 percent by weight silicon, the remainder being iron, plus small amounts of impurities including not over 0.05 percent by weight sulphur and 0.05 percent by weight phosphorus.

6. Steel grit as claimed in claim 5 in which the steel is a tempered martensitic steel which has been drawn back by heat treatment to a hardness of 35 to 47 Rockwell C.

7. Steel grit as claimed in claim 5 in which the steel is a tempered martensitic steel which has been drawn back by heat treatment to a hardness of 47 to 60 Rockwell C.

- References Cited in the file of this patent UNITED STATES PATENTS 

1. FOR USE WITH CENTRIFUGAL BLASTING WHEELS IN THE TREATING OF METAL SURFACES, CRUSHED STEEL SHOT CONSISTING OF STEEL GRIT OF SMALL DIMENSION AND IRREGULAR SHAPE HAVING RELATIVEELY ABRUPT EDGEES AND A HARDNESS WITHIN THE RANGE OF 35 TO 64 ROCKWELL C IN WHICH THE STEEL IS A MARTENSITIC STEEL CONVERTED BY HEAT TREATMENT FROM AN AUSTENITIC STEEL SHOT HAVING FROM 1.00-1.50 PERCEENT BY WEIGHT CARBON, 0.40-0.90 PERCEENT BY WEIGHT SILICON, AND A HARDENING ELEMENT SELECTED FROM THE GROUP CONSISTING OF 1.00-1.50 PERCENT BY WEIGHT MANGANESE, 0.50-5.0 PERCENT BY WEIGHT CHROMIUM, 0.30-5.0 PERCENT BY WEIGHT MOLYBDENUM, 0.05-0.40 PERCENT BY WEIGHTT VANADIUM, 0.50-2.5 PERCENT BY WEIGHT NICKEL, 0.20-1.00 PERCENT BY WEIGHT COPPER AND MIXTURES THEREOF, THE REMAINDER BEING IRON PLUS SMALL AMOUNTS OF IMPURITIES. 